Calculate Weight Distribution for CoG

[Daneel]

Hello!

I'm looking for a program which can calculate the weight distribution (i.e., our RC components) which is necessary to reach a given CoG on a flying wing.

Such a program needs the type of material (e.g. RG20 foam), the used profile and the dimensions of the flying wing as input.

It should then let me play around with the position of the components and the sweep in order to reach a given CoG.

this would me immensly useful for me (and I guess for other builders as well) as it will minimize the usage of lead to get the CoG right.

Is there such a program?

Thanks,
Michael

[capncrunch]

most 3d cad programs can calculate a CG for you, but that's a little bit of overkill.

you can probably do this in a spreadsheet - find the cg of each component, then the distance between a fixed point (say, the center section leading edge) and the CG of each part, and the mass of each part. then you just average all the parts and presto!

for example, if your wing is 5 ounces, and you calculate from its geometry that the CG of the wing is 3 inches back from the LE, and you have 2 servos that total 3 ounces which are 8 inches back from the LE, you have a combined CG that is 8in*3oz + 3in*5oz = (3oz + 5 oz) * X, where X is the distance to the CG of all 8 ounces in the system. X = 4.875" back from the LE.

In the same spreadsheet, you could calculate the MAC from the wing geometry and find the ideal balance point and compare the two.

-barrett

[BMatthews]

For such a program to work well you would need to pick and weigh the wood sheets and input the density of them. Balsa comes in a wider range of density than any other wood and it is this variation that would result in pretty much meaningless CG calculations unless you input specific numbers for the balsa components.

[Sparky Paul]

And the material you sand off doesn't affect the c.g. :)

[feihu]

Daneel -

You really don't need a program to do that. Just use the "summation of moments" applied to a given point which is I chose to be the CG @ 30%MAC.

The intention of this calculation was to find the airplane CG without the battery so that the battery could be located as close as possible to my desired CG, and changeing battery weights would have no effect on the CG.
As it turned out, the completed airplane CG needed only a 1 oz in moment to balance at the desired CG @ 30%MAC.

So, to get the airplane CG, I moved the receiver, elevator and rudder servos to practical locations in the fuselage. Attached is my weight and balance worksheet.

feihu